The present invention relates to a sliding device, utilizing dynamic pressure of a fluid for lubrication, employed for computer peripherals, various types of industrial machinery and machine tools, and so forth, a fluid dynamic pressure bearing to which the configuration of the sliding device is applied, and a motor incorporating the fluid dynamic pressure bearing.
In recent years, a fluid dynamic pressure bearing utilizing dynamic pressure of a fluid for lubrication has been frequently used in place of a conventional roller bearing that is a combination of a rolling member (a ball, a roller, etc.) and a lubricant (lubricating oil, a grease, etc.) as a bearing in a computer peripheral, particularly a motor of a hard disk drive (HDD), a motor of a polygonal mirror composing a laser scanning system in a laser beam printer (LBP), or the like.
FIG. 11 is a cross-sectional view showing a state where a rotating member 1R is rotated in an example of a fluid dynamic pressure bearing 101. FIG. 12 is a perspective view showing the appearance of the fluid dynamic pressure bearing in a state shown in FIG. 11. FIG. 13 is a front view showing the appearance of a fixed member 1S comprising a shaft 102 and flanges 104 in the fluid dynamic pressure bearing 101. FIG. 14 is a plan view showing a sliding surface 104a of the flange 104.
Referring to FIGS. 11 to 13, the fluid dynamic pressure bearing 101 in this example comprises a pillar-shaped shaft 102, a cylindrical sleeve 103 having a through hole 103a, which is circular in cross section, through which the shaft 102 is inserted, and two disk-shaped flanges 104 fixed to both ends of the shaft 102 in such a manner that the sleeve 103 is interposed therebetween with the shaft 102 inserted through the through hole 103a of the sleeve 103.
The two flanges 104 are fixed to both ends of the shaft 102 in a state where the respective sliding surfaces 104a, which are side surfaces opposed to each other, are perpendicular to a central axis X2 of the shaft 102.
The sleeve 103 is formed so that sliding surfaces 103b, which are both end surfaces to which the through hole 103a is opened, opposed to both the sliding surfaces 104a is perpendicular to a central axis X3 of the sleeve 103.
When the fluid dynamic pressure bearing 101 is employed as a bearing in a motor of an HDD, for example, the shaft 102 and the two flanges 104 that are fixed to each other are used as a fixed member 1S with the shaft 102 and the flanges 4 fixed to a member on the fixed side such as a housing of a motor (not shown).
The sleeve 103 is used as a rotating member 1R rotated around a central axis X3 with a member on the rotating side such as a rotor or a magnetic disk of a motor mounted thereon.
Referring to FIGS. 11 and 13, the sleeve 103 is formed such that a distance L3 in the axial direction of the central axis X3 between both the sliding surfaces 103b is slightly smaller than a distance L4 in the axial direction of the central axis X2 between the sliding surfaces 104a of the two flanges 104.
Referring to FIGS. 11 and 14, each of the sliding surfaces 104a of the two flanges 104 is formed as having a plurality of dynamic pressure creation grooves 104b in a herringbone shape for creating dynamic pressure in a thrust direction (a direction of the center axes X2 and X3) in a fluid such as gas or lubricating oil existing between the sliding surfaces 103b and 104a opposed to each other when the sleeve 103 is rotated.
Referring to FIGS. 11 and 13, the sleeve 103 is formed such that the inner diameter D3 of the through hole 103a is slightly larger than the outer diameter D2 of the shaft 102.
A sliding surface 102a, which is an outer peripheral surface of the shaft 102, is formed as having a plurality of dynamic pressure creation grooves 102b in a herringbone shape for creating dynamic pressure in a radial direction in a fluid existing between the sliding surface 102a and an opposed sliding surface 103c, which is an inner peripheral surface of the through hole 103a of the sleeve 103 when the sleeve 103 is rotated.
When the sleeve 103 serving as the rotating member 1R is rotated around the central axis X3, dynamic pressure in a thrust direction is created in a fluid existing between the sliding surfaces 103b and 104a opposed to each other on each of the upper and lower sides in FIG. 11 of the sleeve 103 by the function of the dynamic pressure creation grooves 104b. 
A clearance G34 based on a dimensional difference between the distances L3 and L4 is created between the sleeve 103 and the flanges 104 on each of the upper and lower sides of the sleeve 103, as shown in FIG. 11, on the basis of the dynamic pressure, so that the sleeve 103 rises up to the fixed member 1S in a thrust direction.
In this rotating state, dynamic pressure in a radial direction is created over the whole peripheries of both the members 102 and 103 in a fluid existing between the sliding surfaces 102a and 103c opposed to each other by the function of the dynamic pressure creation grooves 102b. 
A clearance G23 based on a dimensional difference between the diameters D2 and D3 is created over the whole peripheries of both the members 102 and 103 on the basis of the dynamic pressure, so that the sleeve 103 also rises up to the fixed member 1S in a radial direction.
Therefore, the sleeve 103 serving as the rotating member 1R can be rotated in a state where it is lubricated by a fluid with which the clearances G23 and G34 are filled without being entirely brought into contact with the fixed member 1S. By significantly reducing its rotational resistance, therefore, it is possible to reduce a rotation torque produced by the motor as well as lengthen the bearing life.
In the fluid dynamic pressure bearing 101, the dynamic pressure of the fluid is low during a period from the start of rotation to the rotating state shown in FIGS. 11 and 12 and a period from the rotating state to the stop of the rotation. Therefore, the rotating member 1R is rotated in a state where the sliding surfaces 102a and 103c opposed to each other and the sliding surfaces 103b and 104a opposed to each other are respectively brought into direct contact with each other.
Therefore, it is preferable that in the sleeve 103 serving as the rotating member 1R and the shaft 102 and the flange 104 serving as the fixed member 1S, which constitute the fluid dynamic pressure bearing 101, at least the sliding surfaces 102a, 103b, 103c, and 104a of the members 102 to 104 are formed of ceramics in order to improve wear resistance.
When ceramics have insulating properties, however, the rotating member and the fixed member are charged by friction at the time of the rotation in a state where the sliding surfaces opposed to each other are brought into direct contact with each other, so that static electricity is liable to be stored in the fluid dynamic pressure bearing. When the static electricity is stored, a malfunction such as a write error or a read-out error of data may be liable to occur particularly in the HDD.
Both the rotating member and the fixed member composing the fluid dynamic pressure bearing are proposed to be formed or actually formed of conductive ceramics having a small volume resistivity value.
For example, Japanese unexamined patent publication No. 03-75281 (1991) discloses that metal powders are heat-treated in a nitrogen gas atmosphere and nitrided, and are chemically coupled to one another, to form a sliding component such as a bearing composed of conductive reaction sintering ceramics (a metal nitride) having a porosity of 5 to 30% and having a volume resistivity value of not more than 10−3 Ω·cm. It is considered that the configuration is applied to the fluid dynamic pressure bearing.
Japanese unexamined patent publication No. 08-121467 (1996) discloses that a rotating member and a fixed member are formed of ceramics mainly composed of Al2O3 and having a Young's modulus of not less than 300 GPa and having a volume resistivity value of not more than 106 Ω·cm by containing not less than 20 mass % of TiC.
Japanese unexamined patent publication No. 08-152020 (1996) discloses that a rotating member and a fixed member are formed of ceramics mainly composed of an inorganic compound of at least one type selected from a group consisting of SiC, Si3N4, TiC, TiN, and Al2O3—TiC and having a volume resistivity value of not more than 106 Ω·cm by containing 0.1 to 20 mass % of free carbon.
Furthermore, Japanese unexamined patent publication No. 2002-241172 discloses that a rotating member and a fixed member are formed of ceramics mainly composed of an inorganic compound of at least one type selected from a group consisting of Al2O3, ZrO2, and TiN and having a volume resistivity value of not more than 106 Ω·cm by containing a metal oxide, a metal nitride, a metal carbide, a metal boride, or a metal carbonitride each having a conductivity.
When the rotating member and the fixed member are formed of ceramics having a small volume resistivity value as disclosed in the references 1 to 4, however, spark discharges are liable to be induced between the sliding surfaces, opposed to each other, of the rotating member and the fixed member in a state where they are insulated from each other with a fluid such as gas or lubricating oil serving as an insulator interposed therebetween particularly when the rotating member is rotated, thereby making it impossible to continue to stably rotate the rotating member. This problem noticeably arises as the fluid dynamic pressure bearing is miniaturized in conformity with the miniaturization of equipment such as the HDD.
It is considered that this is caused, in handling or operating the equipment such as the HDD, for example, by direct application of a potential difference occurring between a member on the fixed side such as a housing of a motor to which the fixed member is fixed or a case of the HDD to which the housing is fixed and a member on the rotating side such as a rotor or a magnetic disc of the motor to a small clearance between respective sliding surfaces of both members opposed to each other through the rotating member and the fixed member having a small volume resistivity value.
When the equipment such as the HDD is, for example, transported or stored, a packaging material including a cushioning material having insulating properties such as expanded polystyrene is generally employed. In a case where the equipment is packaged by the packaging material and a case where it is taken out of the packaging material, friction is created between the cushioning material and the case of the equipment, whereby the case is liable to be charged.
A charging potential of the case is applied to the sliding surface of the fixed member through the housing of the motor and the fixed member having a small volume resistivity value. When the rotating member is rotated to cause a clearance between the sliding surfaces, opposed to each other, of the rotating member and the fixed member, a potential difference between the sliding surfaces is increased to induce spark discharges.
Even if the volume resistivity values of both members are made small, as described above, both members are still liable to be charged by friction in a state where both members are rotated with the sliding surfaces opposed to each other brought into direct contact with each other when the rotation of the rotating member is started or stopped. When the rotating member is further rotated in such a state where both members are changed to cause a clearance between the sliding surfaces, opposed to each other, of the rotating member and the fixed member, a potential difference between the sliding surfaces is increased to induce spark discharges.
When spark discharges are induced, an axis of rotation of the rotating member moves by a shock caused thereby, making the rotation unstable. Crystal grains at a portion where spark discharges are induced drop out to be particles. The particles enter a small clearance between the sliding surfaces opposed to each other and they are meshed with each other. This may cause the rotation of the rotating member to be further unstabilized or entirely stopped.
Furthermore, the generated particles may be scattered outward from the fluid dynamic pressure bearing through the clearance between the sliding surfaces and adhere to the inside of the equipment such as the HDD to cause a malfunction in the equipment, and electromagnetic waves generated by the spark discharges may induce a malfunction in the equipment.
In the fluid dynamic pressure bearing in which the rotating member and the fixed member are formed of ceramics having a small volume resistivity value, as described in the references 1 to 4, therefore, a measure to prevent spark discharges from being induced is taken by electrically connecting the rotating member and the fixed member to each other.
In recent years, under a mobile environment, the fluid dynamic pressure bearing has been more frequently employed. However, when the rotating member is instantaneously brought into contact with the fixed member while being rotated due to vibration applied to the equipment and is charged by friction, the rotation is stabilized and the rotating member and the fixed member are spaced apart from each other. This induces spark discharges in many recent cases. Therefore, a measure to prevent this problem is required to be taken.